This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to 0122417.9, filed in Great Britain on 17 Sept. 2001; the entire content of which is hereby incorporated by reference.
The present invention is concerned with units and modules used primarily to impart buoyancy to marine objects and secondarily to protect such objects.
Exploration drilling is increasingly taking place in areas of high ocean currents and deep water. When drilling in deep water the suspended weight of the drill string affects the stability of the drillship. The weight also potentially exceeds the tensile strength of the riser string so the majority of the submerged weight of the drill string needs to be balanced off by use of strap on buoyancy modules fabricated from low-density syntactic foam.
During string running/recovery operations, when the drill string is suspended from the drilling rig rather than coupled to the seabed wellhead, subsurface currents act on the drill string to deflect it from vertical. At the same time, surface wave conditions can cause a degree of movement. The drill string moves away from its central position through, and towards the edges of, the rotary table aperture. When the riser joint is xe2x80x9cdressedxe2x80x9d, the energy of the impact between the riser assembly and the rotary table is taken by the syntactic foam, through variable degrees of mechanical damage. The modules provide sacrificial protection for the otherwise vulnerable riser.
During all stages of the operation, the clad riser is also subject to subsurface loop currents. These currents cause significant flexure of the riser joint, with these flexural forces being directly transferred to the buoyancy modules.
The combination of frequent, substantial impacts of the modules with the rotary table plus the high flexural strains applied to the modules by the deflected riser, cracking and full fracture of riser buoyancy modules is now a regular feature of deep water exploration drilling. The repair and replacement of damaged modules has major cost and operational impacts but of far greater importance to the operator are the dropped object hazards to drilling floor personnel and equipment, from break-up of fractured and cracked modules during drill string running and recovery.
The deep water drilling operation demands the use of buoyancy modules of the lowest possible density. Deep-water tolerant, minimum density syntactic foam requires the use of very rigid and therefore relatively brittle resin matrices for the syntactic. It is not commercially of practically viable to utilise flexible, resilient materials. Therefore as relatively brittle materials must be used, there is a requirement for drilling riser modules to incorporate some means whereby the overall structural integrity of the riser module is retained, even after the module has suffered substantial cracking and fracture.
Attempts have been made in the past to provide such residual integrity by means of steel rods or ropes/cables embedded axially in the modules. Both systems can only provide retention of broken sections of modules where the reinforcements actually pass through the broken sections. As there is a practical limit to the number of axial reinforcements that can be included in a module, the residual structural integrity is incomplete. Steel rod reinforcements have the additional disadvantage of increased weigh therefore reduced buoyancy and also create a major safety hazard in the event that the steel rod become exposed due to foam spalling and could potentially spear drilling personnel during riser running and recovery.
An object of the invention is to provide an improved construction for marine buoyancy units and modules.
According to one aspect of the invention there is provided a marine buoyancy unit composed of synthetic plastics material and having an internal cavity for surrounding one or more elongate objects and a mechanically embedded reinforcement layer close to the outermost surface remote from the cavity. According to another aspect of the invention there is provided a marine buoyancy unit composed of syntactic foam and having an internal cavity for surrounding one or more elongate objects and a mechanically embedded reinforcement layer close to the outermost surface remote from the cavity. It has been found that a layer of fibres near the surface maintains residual structural integrity, yet is light enough not to materially affect buoyancy and does not create any new safety hazard.
Preferably the fibres are high strength polymer fibres. Normally the unit is cylindrical and the layer of fibres is installed on the outward facing curvature of the unit and may advantageously be installed in the areas of the choke and kill lines of the module flats. The fibres are made of tough, rather than brittle material and that they possesses significant elongation at break. This elongation performance may be inherent in the fibre e.g. PE, PP, Nylon, Polycarbonate, PET, PE/PP, copolymers or may be achieved in fibres with lower elongation at break by twisting into ropes e.g., xe2x80x9cKevlarxe2x80x9d (RTM). The fibres, which make up the layers are mechanically locked into place within the syntactic foam but should not be tightly bonded in order to allow some limited axial movement in the event of fracture. This is readily achieved by a mesh configuration. The use of a mesh produced from bi-directional bunches of linear fibres embedded in flexible polymer sheathing to which the epoxy syntactic will not bond is preferable.
As cracking failures normally run circumferential in the module, it is preferable, if the mesh system is biased towards greater weight in the axial direction.
As mentioned the mesh is mechanically locked into, rather than bonded to the syntactic foam, it is advantageous but not essential if this mesh is positioned slightly subsurface, preferable 5-25 mm in order to prevent it peeling off the surface after module fracture. This subsurface positioning may be achieved by the use of a thick, open structure lattice material, such as the xe2x80x9cEnkamatxe2x80x9d mesh system from Colbond (icosynthetics Inc of Enka, N.C., USA. The retention system may advantageously be attached to the open structured spacer material, so that it is positioned in the product when the spacer material is attached to the mold.
In selecting the reinforcing fibre the material is requires to be able to tolerate the temperature conditions achieved during product cure, typically a maximum of 150xc2x0 C. and also be resistant to degradation in the marine environment. This latter requirement precludes the use of natural fibres such as hemp and sisal, unless they are encapsulated in a water impervious coating.
A material identified as particularly suitable for the reinforcing fibre is the xe2x80x9cSympaforcexe2x80x9d range of geogrids, manufactured by Synteen Technical fibres Inc of Lancaster, S.C., USA and Synteen GmbH of Klettgau-Erzingen, Germany. This is a high tenicity PET mesh bonded and encapsulated in PVC xe2x80x9cplastisolxe2x80x9d paste. Mesh weights between 100 g/m2 and 750/m2 may be used; with mesh weight being selected to suit the anticipated maximum weight. Meshes with weight and tensile strength biased in the axial direction are preferred, with grades of axial strength of 50-200 kN/m and transverse/circumferential strength of 25-100 kN/m being particularly suitable.
In the event of module fracture due to impact or excessive flexure, the high strength fibre mesh retains all sections of the broken module in nominally the original position. Broken sections are prevented from dropping off the main body of the module, allowing the drilling operation to continue uninterrupted. The broken module can be removed and replaced when production operation allow, typically at the end of the drilling campaign when the riser string is lifted and stored on deck or onshore.
The invention may be understood more readily, and various other features of the invention may become apparent, from consideration of the following description.